The invention relates to electron discharge devices and particularly to photomultiplier tubes having ceramic-metal envelopes.
In U.S. Pat. Nos. 4,376,246 and 4,407,857 issued to G. N. Butterwick on Mar. 8, 1983 and on Oct. 4, 1983, respectively, photomultiplier tubes having ceramic-metal envelopes are described. The Butterwick patents are incorporated by reference herein for disclosure purposes. Ceramic-metal envelopes are sturdier than conventional glass envelopes and are preferred where harsh environmental conditions are encountered. The ceramic insulators used in the envelopes of the photomultiplier tubes described in the above-referenced Butterwick patents comprise a high alumina (Al.sub.2 O.sub.3) ceramic having an alumina content of about 95 percent.
A drawback in using a ceramic insulator in a photomultiplier tube is that the ceramic insulator exhibits electric field induced luminescence. This phenomenon is described in copending U.S. patent application Ser. No. 172,659 filed by McDonie et al. on July 28, 1980, assigned to the assignee of the present invention and incorporated by reference herein for disclosure purpose. In the above-referenced McDonie et al. application, the ceramic insulating support spacers, which support the dynodes and the anode, luminesce in the vicinity of the anode. A coating of high resistance chrome oxide (Cr.sub.2 O.sub.3) with an overlayer of lower resistance Nichrome eliminates the luminescence in the dynode support spacers. However, such a solution is not feasible where the ceramic member comprises a portion of the tube envelope, since the envelope ceramic member must retain its high resistance characteristic and the lower resistance Nichrome overlayer cannot be used.
In the photomultiplier tubes described in the Butterwick patents, referenced above, it has been determined that the ceramic insulator member separating the cathode and stem subassemblies luminesces because of the proximity of the ceramic member to the electron multiplier cage assembly which operates at a maximum voltage of 1800 volts with respect to the cathode.
In an attempt to "quench" this luminescence while retaining the high resistance of the envelope ceramic insulator member, a conventional chrome oxide coating was applied to the ceramic member. The chrome oxide coating was applied as described in the above-referenced McDonie et al. patent application. The interior surface of the ceramic member was sprayed with chrome oxide suspended in a silicate binder. The coated ceramic member was then fired in a water saturated hydrogen atmosphere at 1000.degree. C. for 10 to 15 minutes to cure the chrome oxide. The chrome oxide coated ceramic member was then used to make a ceramic-metal envelope for a tube. Unfortunately, it proved nearly impossible to controllably form a photoemissive cathode on the faceplate of the tube having the chrome oxide coated ceramic member in the ceramic-metal envelope. It is believed that the chrome oxide absorbs large quantities of the alkali metal vapors used to form the photoemissive cathode. The alkali metals significantly reduce the electrical resistance across the ceramic member so that electrical leakage masks the developing sensitivity of the photoemissive cathode. As a result, reproducible photocathodes cannot be made consistently in tubes having ceramic-metal envelopes with a conventional chrome oxide coating disposed on the interior surface of the ceramic member.